Interferometric Measurements of Temperature-Dependent Optical Absorption in Low-Loss SiN Membranes for Laser Sails

We have been evaluating candidate low-optical-loss materials for laser-light sails, which have been proposed as a mode of interstellar travel, and must reflect driving laser light while being highly robust to laser damage. For proposed laser sail missions, absorption coefficients of approximately 10^-2 cm^-1 or lower are required to maintain sail integrity at the highest laser intensities¹. One candidate laser sail material is silicon nitride (SiN) suspended membranes^1,2, which can be formed in different stoichiometries, for example stoichiometric Si₃N₄ and silicon-rich SiN_x (x~1). The low levels of optical loss in such membranes cannot be measured using conventional measurements such as ellipsometry.<br/><br/>In this work, we use self-referencing photothermal common-path interferometry (PCI) to study absorption in thin, free-standing SiN membranes. PCI is a continuous-wave pump-probe technique for measuring optical absorption in low-loss materials³, wherein we interferometrically measure the thermo-optic effect caused by a chopped high-power pump laser (1 W) using a less powerful probe laser (2 mW), with the probe's detector synchronized to the chopper via a lock-in amplifier. We show a new self-referencing technique for PCI by coating the sample with monolayer graphene that has an easily measurable absorbance of the order of 1%, slightly different from the well-known value of 2.3% due to Fabry-Perot effects in the membrane. Using this reference, we demonstrate a self-referencing PCI technique to find absorptivity values in Si₃N₄ and SiN_x (x~1). We found the room-temperature absorption coefficient of Si₃N₄ at 1064 nm to be (2.09 ± 0.76) × 10^-2 cm^-1, and that of SiN_x (x~1) to be 7.94 ± 0.50 cm^-1. These results not only point to the suitability of Si₃N₄ as a candidate material for laser sails, but also show self-referencing PCI as a viable method to measure loss in low-loss, free-standing membranes.<br/><br/>In addition, many dielectrics are known to suffer from increased optical absorption at higher temperatures due to bandgap narrowing, leading to thermal runaway in laser sail models when compounded with two-photon absorption¹. We will present high-temperature absorptivity measurements of Si₃N₄ membranes – using small resistive heaters with feature sizes down to 70 μm – fabricated on top of the membranes using metal evaporation through a shadow mask to achieve localized heating on the membranes only, which avoids introduction of noise to PCI measurements caused by the heating of its optical components.<br/><br/><u>References</u><br/>¹ G.R. Holdman, G.R. Jaffe, D. Feng, M.S. Jang, M.A. Kats, and V.W. Brar, Adv Opt Mater <b>10</b>(19), 2102835 (2022).<br/>² H.A. Atwater, A.R. Davoyan, O. Ilic, D. Jariwala, M.C. Sherrott, C.M. Went, W.S. Whitney, and J. Wong, Nature Materials 2018 17:10 <b>17</b>(10), 861–867 (2018).<br/>³ A. Alexandrovski, M. Fejer, A. Markosian, and R. Route, in <i>Solid State Lasers XVIII: Technology and Devices</i>, (SPIE, 2009), p. 71930D.